Among a power supply unit disposed beneath the floor at the rear of a seat of an automobile, an arrangement in which a battery module is disposed in a lower portion thereof, a DC-DC converter and a motor-driving inverter are arranged side by side in the vehicle width direction in an upper portion thereof, cooling air flowing from the front to the rear of a vehicle body is divided into up and down and cools the lower side battery module and the upper side DC-DC converter and motor-driving inverter in parallel is known from Patent Publication 1 below.    Patent Publication 1: Japanese Patent Application Laid-open No. 2008-062780